The theory is, by providing more targets against the enemy, you make them waste hits while your fighters who are strong enough to matter can pound them for a while as they terminate the useless players. This, effectively, gives your fighters a few seconds of advantage over the enemy, during which, they can pound and take out some of the enemy soldiers; and that can make a big difference; it is equivalent to free damage performed on the enemy. It is, sometimes, the difference between a loss and a win.
MMS players also perform a function of deterrance; the group looks large and threatening, and may cause the enemy to run to get reinforcements. An alert force, however, will "click" on your force to verify levels; anyone who "cons" (short for "consider", a command that estimates the relative level of the target) grey is a decoy, rather like a wooden duck. Or, you might say, a sitting duck.